(a) Field of the Invention
The present invention relates to a crystal layer including conductivity-providing impurities and a method for epitaxially growing the crystal, more in detail, to a technique for adding impurities using a so-called surfactant layer epitaxial technique in which a surfactant layer including a single atomic layer composed of foreign atoms is adsorbed on the surface of a host crystal and in which nucleus atoms arriving on the surfactant layer are sequentially replaced with the surfactant layer to grow the host crystal.
(b) Description of the Related Art
A wide gap semiconductor material such as diamond is greatly expected to be employed for a light emission element and a new device material. A generally employed impurity-addition process such as a thermal diffusion process hardly conducts doping of the impurities providing conductivity to the diamond, and also hardly conducts the formation of an n-type or p-type semiconductor layer thereon having a low resistance (high impurity density) to the diamond. This is because the single n-type or p-type dopant cannot be stably added into a host crystal, and a single n-type or p-type dopant forms a deep level in the host crystal. In order to overcome the problem, a so-called co-doping technique is known in which the host crystal and two different impurities are added. The technique removes instability of an atomic structure resulting from a single element existing alone in the crystal to form a stable and shallow-leveled donor (acceptor) by a three-atom composite of donor (D)-acceptor (A)-donor (D) (or A-D-A) by chemically bonding the two kinds of the impurity atoms.
The co-doping of the diamond is investigated, and a composite including highly soluble nitrogen and boron in diamond as co-dopants (N-B-N) and another composite including phosphorous and highly mobile hydrogen in diamond as co-dopants (P-H-P) are attempted. The co-dopant addition technique supplies a raw material gas having an adjusted partial pressure ratio during the crystal growth to form the composite in the host crystal.
Separately from these, a surfactant layer epitaxy technique is known as a semiconductor crystal growth technique for changing a crystal surface form and for designing a crystal structure. For example, J. M. Lannon Jr., J. S. Gold and C. D. Stinespring, xe2x80x9cEvidence for surfactant mediated nucleation and growth of diamondxe2x80x9d (Appl. Phy. Lett. 73(No.2), pp. 226-228, 1998) introduces a method of epitaxially growing diamond crystals by supplying a C2H4 (acetylene) gas under a ultra high vacuum (4xc3x9710xe2x88x9210 torr.) by employing a silicon layer as the surfactant layer.
Since the co-dopant addition technique conventionally examined simultaneously supplies the two kinds of the dopant raw material gases into a growth gas for forming the host crystal, a probability of forming the co-dopant during the incorporation of the dopant into the host crystal is low, and most of the dopants are incorporated as a single dopant or a two-atom composite. A boron atom which is a p-type dopant and a nitrogen atom which is a n-type dopant form deep levels to obstruct the functions of the target composite dopants, thereby preventing the effective injection of the n-type of p-type carrier into the diamond crystal.
In view of the foregoing, an object of the present is to effectively add three-atom composite dopants having a high density into a host crystal, thereby forming a low resistance semiconductor crystal layer even in a material to which the doping or the co-doping is heretofore difficult to be performed.
The present invention provides a method including the steps of: (1) supplying first impurity atoms onto a surface of a crystal substrate to form a surfactant layer adsorbed on the surface, and (2) supplying nucleus atoms which bond with the first impurity atoms, (3) repeating step (2) until second impurity atoms are supplied in step (4), and (4) supplying second impurity atoms which bond with the first impurity atoms and the nucleus atoms to epitaxially grow a crystal layer including the nucleus atoms as a crystal nucleus material doped with the first and second impurity atoms.
In accordance with the method for growing the crystal, a high density impurity doping can be achieved to provide the crystal having a high stability of a chemical bonding and substantially no crystal deficiency. The present invention enables the addition of the high density dopant even to the material to which the doping or the co-doping is conventionally difficult to be performed. Even the wide-gap semiconductor material may have a lower resistance p-type or n-type layer.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.